1. Field of the Invention
This invention relates to heat exchangers, and particularly to a method of repairing a defective tube in a tube-in-shell heat exchanger, such as a steam generator unit (SGU) of a nuclear reactor.
2. Description of Related Art
A heat exchanger of the tube-in-shell type comprises a shell in which are mounted spaced-apart substantially parallel tubeplates having apertures into which open-ended tubes are welded, so that a bundle of substantially parallel tubes extends between the tubeplates. A first fluid, such as liquid sodium, passes through the shell, in contact with the outside surfaces of the tubes, and a second fluid, such as water, flows through the tubes, so that heat is transferred from one fluid to the other.
In operation of such a heat-exchanger, a leak may develop in a tube. In the case of a liquid sodium cooled reactor, this will allow sodium and steam to mix and to produce a chemical reaction. Neighbouring tubes may be weakened or ruptured by this reaction.
The conventional method of bringing an SGU back into operation following a tube leak is to plug that tube and, for example, at least the neighbouring six tubes, so that all of those tubes become inoperative. This obviously results in a decrease in efficiency of the heat exchanger. Furthermore, because heat is not being extracted from the coolant in the region of the plugged tubes, a hot spot is created in the tube bundle, which can cause stressing of further tubes around the plugged tubes. It will be apparent that only a limited number of leaking tubes and their neighbouring tubes can be plugged before the number of inoperative tubes becomes too large for the SGU to continue to operate.
An alternative method of dealing with leaking tubes, which does not involve plugging, is disclosed in our European Patent No: 0132950. In that method, a short sleeve is inserted into a defective tube through one tubeplate and is explosively welded to that tubeplate and to the inner surface of the tube so that the defect is bridged by the sleeve. This reduces the flow area of the tube, but allows it to remain operative and still contributing to the heat exchange function. Since any hot spots created around these tubes due to the reduced flow will be substantially cooler than if the tubes were plugged, more leaking tubes can be repaired before the SGU has to be finally taken out of service.
However, that method has certain disadvantages. Firstly, the method involves welding on to the tube surface, which might impair the strength of the tube. Secondly, the method is primarily intended for bridging a leaking wed between the tube and the tubeplate, and is less applicable to the bridging of a leak in the wall of the tube itself.